The present invention relates to a brake control system which carries out the so-called ABS control, that is, the brake pressure is controlled so as to prevent wheels from being locked upon braking, and in particular, to a brake control system incorporating a processing function for coping with an abnormality due to disturbance caused by noise imposed on an output from a wheel speed sensor or by inferior installation.
In a brake control system which carries out ABS control, in which a pseudo vehicle body speed is estimated from wheel speeds of wheels, and the brake hydraulic pressure is decreased, held or increased in accordance with the pseudo vehicle speed and the wheel speed so as to prevent any of the wheels from being locked upon braking, in order to allow ensuring of steering ability and running stability to consist with shortening of braking distance.
In such a brake control device, it is important to detect a wheel speed with a high degree of accuracy in order to carry out the above-mentioned ABS control, that is, if a wheel speed sensor becomes abnormal, normal ABS control cannot be carried. Thus, there has been proposed a device for carrying out control coping with an abnormality whenever a wheel speed sensor becomes abnormal.
Such a conventional brake control device is disclosed in Japanese Laid-Open Patent Publication No. H10-147232.
The above-mentioned conventional device includes an abnormality detecting means for detecting an abnormality in each of wheel speed sensors for wheels, and accordingly, if any abnormality is detected, an estimated vehicle speed calculating means calculates an estimated vehicle body speed from wheel speeds of those of the wheels which exclude a wheel as to which the abnormality is detected, and this abnormality detecting means is adapted to determine that a wheel speed calculated in the present control cycle for a given wheel is abnormal, if the vehicle speed calculated in the present control cycle becomes higher than a vehicle speed calculated in a previous control cycle for one and the same wheel, by a value exceeding a set value xcex1, and if it becomes higher than the estimated vehicle body speed by a value exceeding a set value xcex2.
Accordingly, in this conventional brake control device, even though noise is continuously imposed on an output from a wheel speed sensor, the estimated vehicle body speed does not exceed an actual vehicle body speed, and accordingly, it is possible to eliminate occurrence of such disadvantages that unnecessary ABS control is carried out so as to effect unwanted pressure decrease, that erroneous determination is made, that is, an abnormality is determined when a vehicle speed is recovered from a large skid, thereby it is possible to prevent occurrence of pressure decrease caused by unwanted ABS control.
As mentioned above, in the conventional brake control device, even though the degree of control accuracy can be enhanced, since a wheel speed is merely compared with a value which is obtained by adding a previous speed value with the set value xcex1 or xcex2 in order to determine whether an abnormality occurs or not, there is a possibility that occurrence of an abnormality is determined if a drive wheel spins so as to exceed the value, and accordingly, the control coping with the abnormality is sustained even after the spin of the drive wheel vanishes, thereby lowering the control accuracy.
The present invention is devised in view of the above-mentioned problems, and accordingly an object of the present invention is to provide an arrangement capable of detecting an abnormality properly in the case of an abnormality caused by noise imposed on an output from a wheel speed sensor, and discriminating, from such an abnormality, a condition of drive wheel spin, thereby enhancing the control quality.
According to the present invention, a brake control system comprises: a brake unit for reducing brake hydraulic pressures in wheel cylinders for braking wheels of a vehicle, independent from one another; wheel speed sensors for detecting wheels speeds of the wheels, respectively; pseudo vehicle body speed producing means for determining a pseudo vehicle body speed in accordance with outputs of the wheel speed sensors; ABS control means for carrying out ABS control to prevent wheel locking on braking by decreasing a brake hydraulic pressure with the brake unit in accordance with a difference between the pseudo vehicle body speed and each of the wheel speeds; and abnormality determining means for detecting an abnormality in the outputs of the wheel speed sensors, and for causing the pseudo vehicle body speed producing means to determine the pseudo vehicle body speed in accordance with the outputs from the wheel speed sensors, excepting the output of the wheel speed sensor if an abnormality is detected therein. The abnormality determining means comprises an abnormal wheel discriminating means for determining whether an abnormal wheel is a drive wheel or not, to differentiate conditions for resetting the determination of abnormality in dependence on whether the abnormal wheel is a drive wheel or a non-drive wheel.